1. Technical Field
The present invention relates to an image forming apparatus and a gradation correction test chart.
2. Related Art
In image output apparatuses such as color laser printers and the like, it is common that a gradation characteristic is varied due to factors such as temporal variation, environmental variation, individual difference of apparatuses or the like.
As methods for correcting a gradation characteristic varied due to such factors, there are known techniques called “calibration”, “gradation correction” and “color balance correction.”
These techniques are techniques for correcting a gradation characteristic by printing a patch composed of a variety of colors and then comparing the printed patch with a pre-printed reference chart or reading the printed patch by visual observation or with a measuring instrument.
In addition, as methods which can be constructed at low costs without requiring any measuring instrument and reference chart, there have been proposed various methods in which a basis density pattern and a gradation pattern having gradations varied gradually are juxtaposed and printed, shading of the gradation pattern with respect to the basis density pattern is determined by visual observation, and a gradation characteristic is corrected so that an image can be printed at a density close to the basis density pattern.
In addition, for image output apparatuses for reproducing a full color using 3 or more colors such as, for example, Y (yellow), M (magenta) and C (cyan) or Y, M, C and K (black), there is known a calibration method of using a gradation correction test chart in which a band of a basis density pattern as a basis used for printing is arranged in an upper side and several reference density patterns having different densities for each color are arranged in a lower side and the basis density pattern and the reference density patterns of each density are arranged and printed for each color, and a reference density pattern closest to the upper basis density pattern is selected.
At that time, how to make the shading determination as easy as possible was carefully deliberated, such as printing the basis density pattern with a low line number screen strong against density variation (i.e., little density variation) due to the above-mentioned factors, and printing the reference density patterns with a high line number screen used for actual user data printing.
In addition, there is known a method using density contrast patterns of, for example, high density, middle density and low density for each color as one gradation correction test chart in order to provide a precise shading determination and improve a correction precision.
However, in the calibration of the image output apparatuses for color printing, even the gradation correction test chart of one of the above-mentioned types (type of printing a shading contrast pattern of each color on one sheet or type of printing a shading contrast pattern having a plurality of densities for each color on one sheet) could not easily provide a shading determination of the basis density pattern and the reference density patterns for shading contrast patterns of particularly Y (yellow) color among Y, M and C colors or Y, M, C and K color.
This is because the yellow color is greatly varied in a saturation direction for density variation but is little varied in a brightness direction.
In addition, with the yellow color little varied in the brightness direction, since the yellow color is typically printed on a highly bright white recording sheet having a basis density pattern and a density contrast pattern, a shading determination becomes more difficult due to a small brightness difference between the white brightness of the sheet and each pattern of the yellow color.